The present invention relates to a connection construction of a high-voltage resistance wire and more particularly, to a connection construction in which the high-voltage resistance wire is used as an ignition cord in an ignition device for an automotive engine and a terminal to be coupled with a cap, etc. of a spark plug, an ignition coil and a distributor is connected with a winding conductor exposed from a distal end portion of the high-voltage resistance wire such that proper connection between the terminal and the winding conductor is guaranteed for a long term approximately identical with service life of the engine.
In a known high-voltage resistance wire of this kind shown in FIG. 6, a winding conductor 3 having a high electric resistance is prepared by winding around an insulating core 1 a resistance wire 2 formed by a nichrome wire or the like and then, is coated by an insulating coating layer 4 formed by a thick cylindrical rubber member. One distal end of the known high-voltage resistance wire is connected to a terminal as follows. Namely, as shown in FIG. 6(a), the winding conductor 3 and the insulating coating layer 4 are cut to predetermined lengths for cylinder numbers of an automotive engine, respectively. Subsequently, as shown in FIG. 6(b), the insulating coating layer 4 is stripped, through a fixed length from its cut end, from the winding conductor 3 so as to expose a portion of the winding conductor 3. Thereafter, the exposed winding conductor 3 is folded back onto the insulating coating layer 4 as shown in FIG. 6(c) and then, is inserted together with the insulating coating layer 4 into a tubular terminal 5 such that the folded portion of the winding conductor 3 is pressed against an inner bottom surface of the terminal 5 as shown in FIG. 6(d).
In the above mentioned prior art connection method, the folded portion of the winding conductor 3 is inserted into the cylindrical terminal 5 and then, the terminal 5 is crimped so as to be pressed against the winding conductor 3 through the insulating coating layer 4 formed by the large-diameter rubber member. Thus, the resistance wire 2 of the winding conductor 3 is held in close contact with the inner bottom surface of the terminal 5.
However, in this prior art connection method, in case the rubber of the insulating coating layer 4 has been subjected to heat deterioration, elasticity of the rubber of the insulating coating layer 4 drops and contraction of volume of the insulating coating layer 4 takes place, so that a force for bringing the winding conductor 3 into pressing contact with the surface of the terminal 5, which is applied by the insulating coating layer 4, becomes insufficient. In addition, when the terminal 5 and the high-voltage resistance wire are subjected to vibrations and electric discharge, the winding conductor 3 is separated from the terminal 5 and thus, sparks are generated at the separated portion between the winding conductor 3 and the terminal 5, thereby resulting in further deterioration of the rubber of the insulating coating layer 4. Due to these causes, improper contact between the winding conductor 3 and the terminal 5 occurs.
In order to solve the above mentioned conventional problem, various countermeasures for improving contact between the winding conductor 3 and the terminal 5 have been proposed as shown in FIGS. 7(a) and 7(b). In FIG. 7(a), a stopper 5a is provided on the terminal 5 so as to be brought into contact with a folded portion 3a of the winding conductor 3. Meanwhile, in FIG. 7(b), electrically conductive epoxy coating compound 6 is applied to a contact portion between the winding conductor 3 and the terminal 5. Especially, at one distal end portion of the terminal 5 adjacent to the spark plug, which is likely to be subjected to heat deterioration, the countermeasures of FIGS. 7(a) and 7(b) are employed in combination.
Usually, proper connection between the terminal and the winding conductor of the high-voltage resistance wire of this kind is required to be guaranteed for a term approximately identical with service life of the engine of the motor vehicle. However, even if the countermeasures of FIGS. 7(a) and 7(b) are employed in combination, improper contact between the winding conductor and the terminal may happen after running of the motor vehicle for a long period. Namely, in the above mentioned known connection constructions, it is impossible to guarantee connection between the terminal and the winding conductor for a long term.
This drawback of the known connection constructions results from the following factor. Namely, as shown in FIG. 8, since the insulating coating layer 4 is formed by an elastic member, the insulating coating layer 4 having a circular cross section is not brought into point contact with the winding conductor 3 having a circular cross section. However, area of contact of the insulating coating layer 4 with the winding conductor 3 is extremely small. Likewise, since the winding conductor 3 has a circular cross section and the terminal 5 has an annular cross section, the winding conductor 3 is brought into contact, substantially at one point, with the terminal 5.
Furthermore, in the known high-voltage resistance wire having the winding conductor 3, the resistance wire 2 of the exposed winding conductor 3 is readily loosened. If the winding conductor 3 is pressed against the terminal 5 in a state where the resistance wire 2 is loosened, such a problem arises that the resistance wire 2 projects out of the finished product so as to cause leak, thereby resulting in improper ignition of the engine.